The Mutalisk is the primary Zerg air unit. It's fast, has a decent attack and is able to hit land and air.

Early-game Mutalisks are Harassment units and should be used as such. They should only directly engage an army when they massively outnumber the opposing force or are supporting a large ground force. Their strength lies in their mobility, picking off stray units, workers, and buildings. Because few units are fast enough to keep up with Mutalisks, they provide indirect map control - if an enemy balls up their forces to take the offensive, Mutalisks can be used to pick off buildings and units within the enemy base, forcing the enemy back to defend.